Understanding rare molecule could enable “rechargeable heat battery”

A molecule of fulvalene diruthenium, which changes its configuration when it absorbs heat, and later releases heat when it snaps back to its original shape (Image: Jeffrey Grossman)

In figuring out how a molecule called fulvalene diruthenium works to store and release heat, researchers at MIT may have paved the way for a rechargeable battery that stores heat instead of electricity. Although the molecule was discovered in 1996, ruthenium’s rarity and cost has ruled out it’s widespread use but the researchers say understanding the fundamental mechanism of how the molecule works should make it possible to find similar chemicals based on more abundant, less expensive materials.

When it absorbs sunlight, fulvalene diruthenium undergoes a structural change, putting it into a higher-energy state in which it can remain indefinitely. With the addition of a small amount of heat or a catalyst, it snaps back to its original shape, releasing heat in the process. But the tem found an intermediate step that plays a major role in the process whereby the molecule forms a semi-stable configuration partway between the two previously known states.

Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in the Department of Materials Science and Engineering says it is this unexpected two-step process that helps explain why the molecule is so stable and why the process is easily reversible and also why substituting other elements for ruthenium has not worked previously.

In effect, explained Grossman, this process makes it possible to produce a “rechargeable heat battery” that can repeatedly store and release heat gathered from sunlight or other sources. In principle, Grossman said, a fuel made from fulvalene diruthenium, when its stored heat is released, “can get as hot as 200 degrees C (392 F), plenty hot enough to heat your home, or even to run an engine to produce electricity.”

Compared to other approaches to solar energy, he said, “it takes many of the advantages of solar-thermal energy, but stores the heat in the form of a fuel. It’s reversible, and it’s stable over a long term. You can use it where you want, on demand. You could put the fuel in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge.”

Now that the process is understood it should be easier to find other materials that exhibit the same behavior. The researchers now plan to use a combination of simulation, chemical intuition and databases of tens of millions of known molecules to look for candidates that have similar structural similarities and might exhibit the same behavior.

“It’s my firm belief that as we understand what makes this material tick, we’ll find that there will be other materials” that will work the same way, Grossman said.

The MIT team’s findings were reported recently in a paper published in the journal Angewandte Chemie.

Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag. All articles by Darren Quick

Nearly a year later and no follow up on any progress, I can only assume that lack of published papers means no joy yet. I would be interested If Gizmag knows of any recent interviews with the researchers either online or in print.

As an ex-fuel cell researcher and follower of anything to do with energy I have always dreamed of something like this. What a shame it has to be a ruthenium compound! Chemistry can be so confoundedly un-accommodating. Like platinum and palladium being the only effective catalysts for fuel cells, with silver and gold only second best. Lets hope they succeed in finding a much cheaper alternative.